Polyesters typified by polyethylene terephthalate, polybutylene terephthalate and polyethylene naphthalate excel in mechanical and chemical properties, and are used in a wide variety of fields including fibers for clothes and industrial materials, films or sheets for packaging materials or magnetic tapes, bottles, which are hollow molded articles, casings of electric or electronic appliances, and other types of molded articles or components.
Certain representative polyesters, namely, polyesters composed of aromatic dicarboxylic acid components and alkylene glycol components as major constituents, such as polyethylene terephthalate, are produced by first preparing bis(2-hydroxyethyl)terephthalate (BHET) and oligomers containing the same by an esterification reaction between terephthalic acid and ethylene glycol or transesterification of dimethyl terephthalate and ethylene glycol, and then subjecting them to melt-polycondensation in vacuo at high temperatures in the presence of a polycondensation catalyst.
As such a polycondensation catalyst for producing polyesters, antimony trioxide is heretofore widely known. Antimony trioxide is a catalyst which is inexpensive and is of excellent catalytic activity, however, it has some problems. For example, antimony metal is precipitated during polycondensation of raw materials for polyester thereby making the resulting polyester darkened, or the resulting polyester is contaminated with foreign substances.
Thus, it is known that such an alkali as sodium hydroxide or potassium hydroxide is made to be present in a reaction system together with a catalyst in production of polyester so that coloring of the polyester obtained may be prevented (see a patent literature 1). Also in the case of an antimony trioxide catalyst, it is known that when it is used together with a certain amount of sodium oxide and iron oxide, the color tone of polyester obtained can be improved (see a patent literature 2). However, in recent years, as antimony trioxide is inherently poisonous, development of a catalyst free of antimony has been awaited.
Under such a situation, as a polycondensation catalyst for producing polyesters by ester interchange of dimethyl terephthalate and ethylene glycol, there are proposed, for example, glycol titanate (see a patent literature 3), and tetraalkoxy titanium (see a patent literature 4). In recent years, it is proposed to use as a polycondensation catalyst a solid titanium compound which is obtained by hydrolyzing a titanium halide or a titanium alkoxide to obtain hydroxides of titanium, and then heating the hydroxides at a temperature of 30-350° C. to dehydrate and dry the same (see patent literatures 5 and 6).
The titanium catalysts as described above have in many cases a high polymerization activity, but on the other hand, the polyesters obtained using such titanium catalysts are found to be colored yellow, and have problems in that they are readily degraded and colored when they are melt-molded, as well as they are inferior to transparency.
In order to solve such problems as mentioned above, a catalyst which is obtained by adding an aqueous solution of titanium tetrachloride and an aqueous solution of sodium hydroxide simultaneously to an aqueous slurry of particles of a solid base such as magnesium hydroxide or hydrotalcite to hydrolyze titanium tetrachloride to form a coat layer of titanic acid on the surfaces of the particles of the solid base is proposed (see a patent literature 7). However, also in the production of polyesters using such a catalyst, the polyesters obtained have hue which still must be improved. In addition, when polycondensation is carried out at a high temperature and under a reduced pressure, it is found that the polyester formed in part degrades by side reactions possibly due to the catalyst used.    [Patent literature 1] JP-B-S38-2143    [Patent literature 2] JP-A-H09-291141    [Patent literature 3] JP-B-S46-3395    [Patent literature 4] JP-A-S49-57092    [Patent literature 5] JP-A-2001-64377    [Patent literature 6] JP-A-2001-114885    [Patent literature 7] JP-A-2006-188567